Design Issue: Static vs Non-static instance variables

Question

I am designing an Address Book and in order to make my AddressBookApp class work (which includes my main method) I have had to create instance variables and make them static in order for each method in my class to be able to access my Name, Email, and Phone objects. I assume that there is a better way, but am struggling to know what that is. Should I create the objects in the main method? Are instance variables the right way to go? Do you guys have any ideas as to how I can improve my design? (If you have any other design suggestions not related to my question then let me know)

Here is the code for my AddressBookApp class:

import java.util.Scanner;

public class AddressBookApp {

//Instance Variables
private static Name name;
private static Email email;
private static Phone phone;

//Constructor
public AddressBookApp() {
    name = new Name();
    email = new Email();
    phone = new Phone();
}

//Main method
public static void main(String[] args) {
    new AddressBookApp();

    System.out.println("Welcome to the Address Book Application\n");
    Scanner sc = new Scanner(System.in);

    int menuNumber;
    do {
        menu();

        menuNumber = sc.nextInt();
        System.out.println();

        if (menuNumber < 1 || menuNumber > 4){
            System.out.println("Please enter a valid menu number\n");
        } else if (menuNumber == 1) {
            printEntries();
        } else if (menuNumber == 2) {
            addEntry();
        } else if (menuNumber == 3) {
            removeEntry();
        } else {
            System.out.println("Thanks!  Goodbye.");
            sc.close();
            return;
        }

        continue;

    } while (menuNumber != 4);
    sc.close();
} 

/**
 * Prints out Main Menu
 */
public static void menu() {
    System.out.println("1 - List entries\n" + 
                       "2 - Add entry\n" +
                       "3 - Remove entry\n" +
                       "4 - Exit\n");

    System.out.print("Enter menu Number: ");
}

/**
 * Prints all entries in the Address Book
 */
public static void printEntries() {
    name.printNames();
    System.out.println();

    email.printEmails();
    System.out.println();

    phone.printPhoneNumbers();
    System.out.println();
}


/**
 * Adds an entry to the Address Book
 */
public static void addEntry() {
    Scanner sc = new Scanner(System.in);

    System.out.print("Enter Name: ");
    name.addName(sc.nextLine());

    System.out.print("Enter Email Address: ");
    email.addEmail(sc.nextLine());

    System.out.print("Enter Phone Number: ");
    phone.addPhone(sc.nextLine());

    System.out.println("\nRecord Saved.\n");
}

/**
 * Removes and entry from the Address Book
 */
public static void removeEntry() {
    Scanner sc = new Scanner(System.in);

    System.out.print("Please Enter the record number that you would like to remove: ");

    int records = sc.nextInt();
    name.removeNames(records - 1);
    email.removeEmail(records - 1);
    phone.removePhone(records - 1);
}
}

If you're just looking for general comments on your design, you should ask at codereview.stackexchange.com. — Oliver Charlesworth
– Oliver Charlesworth, Commented Jan 4, 2013 at 21:16
"I have had to create instance variables and make them static" - um, what? An instance variable isn't static, by definition. It's associated with a specific instance. — Jon Skeet
– Jon Skeet, Commented Jan 4, 2013 at 21:17
Also, as a matter of terminology, static is the opposite to instance variables. — SJuan76
– SJuan76, Commented Jan 4, 2013 at 21:17
Your question is riddled with contradictions and false assumptions. There is indeed a better way. Stop making things static. Start by assuming that the only thing that needs to be static is main(), and only make other things static when you know you need to. You're proceeding on the opposite assumption. — user207421
– user207421, Commented Jan 4, 2013 at 21:20
@Kebven You only thought you had to do that because you made another mistake and made the methods static too. Don't do that. — user207421
– user207421, Commented Jan 4, 2013 at 21:26

Sotirios Delimanolis · Accepted Answer · 2013-01-08 22:16:28Z

4

AddressBook and AddressBookApp should be two different classes. AddressBook should look like this:

public class AddressBook {

//Instance Variables
private Name name;
private Email email;
private Phone phone;

//Constructor
public AddressBook() {
    name = new Name();
    email = new Email();
    phone = new Phone();
}

// more Constructors

public void setName(Name name) {
    this.name = name
}

public Name getName() {
    return name;
}
// more getters and setters

Your app can then create an instance of this in your main() method:

AddressBook book = new AddressBook();
book.setName(new Name("Jeff"));
//more operations on book

You can pass around the object book to any methods in which you need it or keep creating new instances. You can also have it as a static reference in your app class:

private static AddressBook book = new AddressBook();

// in your app class methods
book.burnBeforeReading();

edited Jan 8, 2013 at 22:16

answered Jan 4, 2013 at 21:19

Sotirios Delimanolis

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Comments

asgoth · Accepted Answer · 2013-01-04 21:23:20Z

0

Simple, create two classes: Main and AddressBook.

Main only has

public static void main(String[] args) {

new AddressBook().execute();
...

AddressBook only has instance methods.

answered Jan 4, 2013 at 21:23

asgoth

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1 Comment

neuquen Over a year ago

Thanks! You were right too, but Sotirios beat you to it so I'm afraid he gets the check.

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